Gravitational Lensing as a Probe of Cold Dark Matter Subhalos
Figure 1
Weak and strong lensing. (a) Weak lensing occurs when the lens (here illustrated by a gray elliptical galaxy, surrounded by a dark matter halo which extends to the outer circle) lies relatively far from the line of sight between the observer (eye) and the background light source (star). In this case, where the sightline misses the central galaxy, only a single image is produced, subject to mild magnification and distortion. The signatures of this are only detectable in a statistical sense, by studying the weak lensing effects on large numbers of background light sources. (b) Strong lensing can occur when the dense central region of the lens galaxy is well-aligned with the line of sight. The light from the background light source may then reach the observer along different paths, corresponding to separate images in the sky. This case is also associated with high magnifications and strong image distortions. The angular deflection in this figure, as in all subsequent ones, has been greatly exaggerated for clarity.
